1. Field of the Invention
This invention relates to a control system for a gas-turbine engine, more specifically to a control system for a stationary gas-turbine engine.
2. Description of the Prior Art
A recent trend in gas-turbine engines, particularly stationary gas-turbine engines, is the development of small turbines for driving the relatively small-output generators of independent power plants. Such plants are coming into wide use in areas where public power service is nonexistent or unreliable.
Gas-turbine engines of this type generally use a gas fuel, either natural gas or liquefied gas. Typical examples include methane, ethane, propane, butane and the like. For improved emission performance, the combustion temperature (adiabatic flame temperature) of such gas-turbine engines is preferably controlled to fall within the prescribed range indicated by the reference symbols a and b in FIG. 13 so as to reduce NOx, CO and other harmful emissions.
Emission performance is also affected by the combustion conditions. In particular, the level of NOx emission rises with increasing combustion temperature. Although premix combustion is advantageous from the aspect of emission performance, because combustion proceeds with uniform temperature distribution, the combustion is liable to be unstable or experience flame-out. On the other hand, diffusive combustion is stable but includes scattered high-temperature regions that increase NOx emission.
Therefore, when the combustion temperature is in a particular region of the temperature range indicated in FIG. 13, specifically when it is above the lower limit value indicated by the reference symbol a, premix combustion is preferable from the aspect of emission performance but is disadvantageous depending on the operating condition because, during idling, for example, the instability of premix combustion may lead to flame-out. A need is therefore felt for a gas-turbine engine of the type under discussion that is capable of operating stably in response to load demand, without flame-out, while achieving excellent emission performance.
Use of natural gas poses a special problem. Since some localities do not set a standard regarding natural gas constituents, the composition of the supplied gas (fuel composition) may vary. Variation in gas composition causes variation in the amount of heat produced by the combustion.
When gas of a constant composition is unavailable, flame-out can be avoided by setting the lower limit value a for switching from diffusive combustion to premix combustion relatively high. But this solution has the drawback that emission performance declines in proportion to the reduction of the premix combustion operating region.
The problem caused by variation in gas composition arises because, even when the gas supply rate is maintained constant, the combustion temperature varies owing to fluctuation in the amount of heat produced by the combustion. Monitoring the gas composition is, however, substantially impossible.
Further, as taught by Japanese Laid-Open Patent Application No. Hei 1 (1989)-163426, a multiplex venturi mixer composed of a plurality of individual venturi mixers has been developed., which mixes the gas fuel with the sucked air with the mixers. When the conventional detection method is applied to this type of multiplex venturi mixers, the throat pressure of the individual venturi mixers must be detected. This complicates the structure owing to the need for a large number of sensors and the like.
Furthermore, when this type of multiple venturi mixer is used as shown, the flow rates of the individual mixers may inevitably be different from each other due to the manufacturing variance, aging and some similar factors. Even if the mixers are manufactured with careful attention to the size, the mixer outlet pressure, etc. will cause the individual flow rates to vary, making it difficult to control the supply of gas fuel or air/fuel ratio accurately.
A first object of this invention is therefore to overcome this problem by providing a control system for a gas-turbine engine that uses a gas fuel such as natural gas, which, when operated using a gas fuel whose composition is not constant, can avoid flame-out and operate stably in response to load demand while also achieving excellent emission performance.
A second object of this invention is therefore to overcome this problem by providing a control system for a gas-turbine engine that uses a multiple venturi mixer, which can control the fuel supply, without the need to detect the throat pressure of each of the multiple venturi mixer.
A third object of this invention is therefore to overcome this problem by providing a control system for a gas-turbine engine that uses a multiple venturi mixer, which can control the fuel supply, even when suffered from the influence of the mixer outlet pressure and some similar factors.
For realizing the first object, the present invention provides a system for controlling a gas-turbine engine having a combustor which is supplied with air drawn in and compressed by a compressor and gas fuel supplied from a gas fuel supply source and which generates a combustion such that resulting combustion gas rotates a turbine that is connected to the compressor and a load to drive the compressor and the load, comprising: fuel regulating means for regulating a flow rate of the gas fuel to be supplied to the combustor; air flow rate detecting means for detecting a flow rate of the air to be supplied to the combustor; oxygen concentration sensor for detecting oxygen concentration of the resulting combustion gas; and calorific value calculating means for calculating a calorific value generated by the combustion in the combustor based on at least the detected flow rate of the air and the oxygen concentration; wherein the fuel regulating means regulating the flow rate of the gas fuel to be supplied to the combustor based on the calculated calorific value.
For realizing the second object, the present invention provides a system for controlling a gas-turbine engine having a system for controlling a gas-turbine engine having a combustor which is supplied with an air-fuel mixture made up of air drawn in through an air intake and supplied through an air passage while being compressed by a compressor and gas fuel supplied through a fuel supply passage from a gas fuel supply source and which generates a combustion such that resulting combustion gas rotates a turbine that is connected to the compressor and a load to drive the compressor and the load, comprising: fuel regulating means provided at the fuel supply passage for regulating a flow rate of the gas fuel to be supplied to the combustor; a venturi tube having an inlet end connected to the air passage and an outlet end opened into the combustor, the venturi tube having a throat of a predetermined sectional area at a location between the inlet end the outlet end; gas fuel jetting means having an inlet end connected to the fuel supply passage at a location downstream of the fuel regulating means and an outlet end connected to the throat of the venturi pipe, the gas fuel jetting means having an orifice of a predetermined opening area which jets the gas fuel supplied from the fuel supply passage into the air passing the throat to form the air-fuel mixture to be supplied to the combustor; gas fuel mass flow rate calculating means for calculating a mass flow rate of the gas fuel passing through the orifice; gas fuel temperature detecting means for detecting a temperature of the gas fuel; gas fuel pressure detecting means for detecting a pressure of the gas fuel; venturi inlet air temperature detecting means for detecting an inlet temperature of the air flowing into the venturi pipe; venturi inlet air pressure detecting means for detecting an inlet pressure of the air flowing into the venturi pipe; air mass flow rate calculating means for calculating a mass flow rate of the air passing through the throat based on the calculated mass flow rate of the gas fuel, the detected gas fuel temperature and the pressure, the detected inlet air temperature and the pressure, the predetermined sectional area of the throat, and the predetermined opening area of the orifice; and fuel supply control means for controlling supply of the gas fuel through the fuel regulating means based on the calculated flow rates of the gas fuel and the air.
For realizing the third object, the present invention provides a system for controlling a gas-turbine engine having a combustor which is supplied with an air-fuel mixture made up of air drawn in through an air intake and supplied through an air passage while being compressed by a compressor and gas fuel supplied through a fuel supply passage from a gas fuel supply source and which generates a combustion such that resulting combustion gas rotates a turbine that is connected to the compressor and a load to drive the compressor and the load, comprising: fuel regulating means provided at the fuel supply passage for regulating a flow rate of the gas fuel to be supplied to the combustor; a venturi tube having an inlet end connected to the air passage and an outlet end opened into the combustor, the venturi tube having a throat of a predetermined sectional area at a location between the inlet end the outlet end; gas fuel jetting means having an inlet end connected to the fuel supply passage at a location downstream of the fuel regulating means and an outlet end connected to the throat of the venturi pipe, the gas fuel jetting means having an orifice of a predetermined opening area which jets the gas fuel supplied from the fuel supply passage into the air passing the throat to form the air-fuel mixture to be supplied to the combustor; and fuel supply control means for controlling supply of the gas fuel through the fuel regulating means; wherein a ratio of the predetermined sectional area of the throat and the predetermined opening area of the orifice is set to a predetermined value.